Ultraviolet irradiation to a semiconductor generates electrons having a strong reduction activity and positive holes having a strong oxidation activity, to decompose a molecular species that comes in contact with the semiconductor by an oxidation-reduction activity. Such an activity is called a photocatalytic activity. By the photocatalytic activity, bad-smelling substances in a living or working space are decomposed and removed, and substances such as organic solvents, agrochemicals and surfactants in water are decomposed and removed. As a substance showing the photocatalytic activity, titanium oxide is attracting much attention and photocatalysts made of titanium oxide are in the market.
However, the photocatalytic activity shown by the photocatalysts made of titanium oxide available in the present market is not sufficient when the photocatalysts are irradiated with visible light.